Hydraulically driven traveling vehicle

ABSTRACT

A hydraulically driven traveling vehicle comprising a pair of right and left driving axles (driving sprockets) differentially connected to each other by a differential mechanism, the input section of the differential mechanism having the output rotation of traveling HST  110  ( 110 ) transmitted thereto, both driving sprockets having mutually opposite two-flow output rotations separately transmitted thereto from a steering HST ( 120 ), thereby effecting traveling driving and turning, electromagnetic solenoids ( 61   a,    61   b,    62   a,    62   b ) serving as output regulating devices for the traveling HST and the steering HST being provided so that the manipulated variables and directions of a speed change lever and a steering operation tool are converted into electric signals, on the basis of which electric signals the output current values of the electromagnetic solenoids are controlled, causing the respective output rotary speeds of the HSTs to correspond to the output current values.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP00/09063, filed Dec. 20, 2000, whichclaims priority to Japanese Patent Application No. 2000-26377, filedFeb. 3, 2000 and Japanese Patent Application No. 2000-26378, filed Feb.3, 2000. The International Application was published under PCT Article21(2) in a language other than English.

TECHNICAL FIELD

The invention relates to a hydraulically driven traveling vehicle e.g.,a crawler vehicle provided with a pair of right and left crawlertraveling devices, having respective driving axles differentiallyconnected to each other through a differential mechanism, wherein poweris inputted into a traveling hydrostatic stepless transmission(hereinafter, “a traveling HST”) so as to drive the right and leftcrawler traveling devices for traveling of the vehicle, and whereinpower of a steering hydrostatic stepless transmission (hereinafter, “asteering HST”) is inputted to the differential mechanism so as to make adifference of rotary speed between the left and right driving axles(left and right driving sprocket shafts, if the vehicle is a crawlervehicle) for turning of the vehicle.

BACKGROUND ART

International Publication WO98/12098, for example, discloses awell-known vehicle provided with a pair of right and left crawlertraveling devices having respective drive axles differentially connectedto each other through a differential mechanism, wherein power isinputted to a traveling HST so as to drive right and left crawlertraveling devices for traveling of the vehicle, and wherein power of thesteering HST is inputted to the differential mechanism so as to make adifference of rotary speed between the right and left crawler travelingdevices for turning of the vehicle.

In the vehicle, the traveling HST and the steering HST are provided withrespective hydraulic servomechanisms for controlling positions ofrespective movable swash plates. Each of the servomechanisms includes anelectronically automatic control valve and a manipulated control valve.The manipulated control valve of the traveling HST is interlockinglyconnected through a mechanical linkage to a lever (or pedal, etc.) forspeed change, and that of the steering HST to a steering handle (asteering operation tool in this document, however, it may be a lever,etc.). Such manipulation devices are manipulated so as to control themanipulated control valves, thereby controlling the positions of themovable swash plates, respectively. It can be read in the document thatthe automatic control valves are provided for adjusting the positions ofthe respective movable swash plates controlled by manipulation.

Many of such constructed vehicles use the traveling HST as a main speedchange mechanism and have a multi-stage sub speed change mechanismincluding a plurality of speed changing gears or hydraulic clutches,which serves as a transmission system interposed between the travelingHST and the differential mechanism. In this case, the above-mentionedspeed change lever for controlling output of the traveling HST serves asa main speed change lever. Additionally, a sub speed change operationdevice (like a lever or a switch) is provided separately from the mainspeed change lever so as to select a speed stage of the sub speed changemechanism. The vehicle disclosed in the cited document also has such astructure.

If output of the traveling HST and output of the steering HST arecontrolled individually, i.e., if the movable swash plate of thetraveling HST is controlled independently of manipulation of thesteering handle, or if the movable swash plate of the steering HST iscontrolled independently of manipulation of the speed change lever, thevehicle turns at the speed set by the speed change lever regardless ofthe manipulated degree of the steering handle. Therefore, the vehicledoes not decelerate even if the steering handle of the vehicle travelingfast is turned to the limit. Such a turning is unstable and may bedangerous.

It is assumed that output of the steering HST is decided uniformlyaccording to the steering angle of the handle regardless of the settingof the speed change lever. Even if the steering angle of the handle isconstant, a ratio of output of the traveling HST to that of the steeringHST becomes large in high-speed traveling, thereby making a turningradius large. On the contrary, in low-speed traveling, the ratio becomessmall, thereby making the turning radius small. In this way, sense insteering operation and accuracy in turning of the vehicle become wrong.

The output of the steering HST is used for acceleration of aturning-outside crawler traveling device and deceleration of aturning-inside crawler traveling device, thereby turning the vehicle.The larger the steering angle becomes, the smaller the driving speed ofthe turning-inside crawler traveling device becomes, and it reacheszero. If the steering angle is further increased, the rotation of theturning-inside crawler traveling device is reversed. Such a turningwhile the turning-inside crawler traveling device is stationary orrotated reversely is called a brake turn. If the output of the steeringHST is independent of the output of the traveling HST as mentionedabove, the steering angle at the time of start of the brake turn, i.e.,when the turning-inside crawler traveling device stops, varies accordingto variation of output of the traveling HST. Thus, a driver mustmanipulate the steering handle to adjust the angle thereof troublesomelywhen the brake turn is going to be done.

Then, in the vehicle disclosed in the cited document, the linkagebetween the speed change lever and the manipulated control valve forcontrolling the position of the movable swash plate of the traveling HSTis mechanically interlocked with the linkage between the steering handleand the manipulated control valve for controlling the position of themovable swash plate of the steering HST, so that the traveling speedvaries correspondingly to the steering angle of the handle. Basically,the larger the steering angle becomes, the smaller the output of thetraveling HST is made so as to decrease the real vehicle-center speed.Furthermore, the larger the stroke of the speed change lever becomes(toward the maximum speed limit), the larger the deceleration degreebecomes, so that, when the steering angle is adjacent to its maximum,the real vehicle-center speed is almost constant regardless of thesetting position of the speed change lever.

However, in this interlocking structure, while the output of thetraveling HST varies correspondingly to the position of the speed changelever and the steering angle of the handle, the output of the steeringHST varies correspondingly to only the steering angle. That is, whilethe steering angle is constant, the output of the steering HST isconstant whether the output of the traveling HST increases or decreases,so that the ratio of output between the steering HST and the travelingHST at any steering angle varies according to the position of the speedchange lever. Thus, it is impossible to make the turning radius at everyarbitrary steering angle exactly constant however the position of thespeed change lever, i.e., the traveling speed varies. Further, theabove-mentioned object, that is, to make the steering angle at thestarting of brake turn constant, is not achieved.

It is unfit for a small crawler vehicle to further improve the citedmechanical linkage for achievement of the above objects, because theimprovement further complicates the linkage, expands a space for it, andincreases costs.

The cited vehicle travels at approximately constant speed while thesteering angle is extremely small. However, if the steering angleexceeds a certain value, the traveling speed reduces proportionally.Thus, for example, the reduction ratio of traveling speed to theincrease of steering angle, which is good when the steering angle islarge to some degree, is felt too large when the steering angle issmall. Namely, when the steering angle is not so large and even if it ischanged a little, a great reduction of traveling speed is felt and aturning circle of the vehicle becomes too small.

In this way, mechanical connection of output of the traveling HST withoutput of the steering HST is necessarily inaccurate in control, or itmust be further complicated for enhancing the accuracy. Such acomplicated structure is not acceptable with respect to an arrangementspace and costs.

Furthermore, for stopping such a vehicle having the HSTs, instead ofdisengagement of a clutch, the speed change lever is set to the neutralposition so as to stop output of the traveling HST, the steering handleis returned to the straight traveling position so as to stop output ofthe steering HST, and then, a brake is actuated so as to stop the leftand right axles surely. However, when the brake must be hit, neither thetraveling HST nor the steering HST is put into neutral, therebyadvancing wearing of the drive axles. Moreover, if the neutralization ofthe traveling HST and the steering HST after braking is forgotten, theoutputs of both the HSTs rebound suddenly when the brake is released.However, it is troublesome to put both the speed change lever and thesteering handle into the respective neutral positions every brakingoperation.

On the other hand, it is hard for a driver to comprehend therelationship between the position of the speed change lever and thetraveling speed because the speed change lever changes the travelingspeed steplessly. Therefore, it is difficult to re-create the travelingspeed after the speed change lever is returned to the neutral positionfor stopping the vehicle.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a vehiclecomprising a pair of left and right drive axles (e.g., left and rightdriving-sprocket shafts of respective crawler traveling devices)differentially connected to each other through a differential unit, atraveling HST whose output rotation is transmitted to an input sectionof the differential unit for traveling of the vehicle, a steering HSTwhose mutually opposite two-flow output rotations are transmitted to therespective drive axles for turning of the vehicle, and tools manipulatedby a driver including a speed change operation tool for setting atraveling speed in each of forward and backward travelings and asteering operation tool (e.g., a steering wheel) for setting a turningradius in each of left and right turnings, wherein the vehicle does notrequire a complicated mechanical linkage but has a compact, simple andeconomical structure which can control such outputs of the traveling HSTand the steering HST as to ensure a good feeling in turningcorrespondingly to manipulation of the speed change operation tool andthe steering operation tool.

To attain the first object, according to the present invention,electromagnetic solenoids serve as respective output regulating means ofthe traveling HST and the steering HST. The degrees and directions ofthe speed change operation tool and the steering operation tool areconverted into electric signals, and electric output currents of theelectromagnetic solenoids are controlled based on the electric signalsso as to realize respective output rotational speeds of the HSTscorresponding to the output currents. Each of output currents of theelectromagnetic solenoids of the traveling HST and the steering HST iscontrolled based on both the electric signal caused by manipulating thespeed change operation tool and the electric signal caused bymanipulating the steering operation tool.

Output of each HST is not controlled by interposing a mechanical linkagefrom the speed change operation tool and the steering operation tool tothe traveling and steering HSTs, but is controlled by controlling theoutput current value of the electromagnetic solenoid according toelectric data with respect to positions of the speed change operationtool and the steering operation tool, whereby it is achieved by asimple, compact and economic structure.

For such a simple and compact structure, a hydraulic servomechanism mayserve as each of the output regulating means of the traveling HST andthe steering HST. An electromagnetic proportional valve serving as meansfor hydraulically controlling the servomechanism may be provided withthe above-mentioned electromagnetic solenoid.

For improving the feeling in turning, the output current value of theelectromagnetic solenoid of the traveling HST is controlled in theabove-mentioned way while the steering operation tool is operated fromthe straight traveling setting position to a limit setting position foreither left or right turning so that the output speed of the travelingHST during straight traveling of the vehicle is little changed while thesteering operation tool is within a certain range from the straighttraveling position, and that it is reduced in a geometric progressionwhen the steering operation tool is over the certain range. In this way,while the steering angle is small, the traveling speed is little reducedso that the vehicle can turn a little at almost the same speed with thatin straight traveling. If the steering angle becomes large, thetraveling speed is reduced in geometric progression so that the vehiclecan do a small turn or a brake turn safely and comfortably.

While a speed of the drive axle on inside in turning is reduced duringmanipulation of the steering operation tool from the straight travelingposition to reduce the turning radius according to the above-mentionedcontrol of output currents of the respective electromagnetic solenoidsof the traveling HST and the steering HST, a setting position of thesteering operation tool when the speed becomes zero is fixed so that thesteering angle on starting of the brake turn is fixed, thereby enablinga driver to do steering operation based on exact notice of the steeringangle for starting the brake turn. Thus, a driver is prevented frombeing shocked by unexpectedly early brake turn during the steeringoperation.

The output current of the electromagnetic solenoid of the traveling HSTis so controlled that a ratio of output speed of the traveling HST whenthe steering operation tool is set at any arbitrary position to theoutput speed thereof when the vehicle travels straight is fixed whereverthe speed change operation tool is set. Accordingly, whichever settingposition the speed change operation tool is put into, a speed of thevehicle-center when the steering operation tool is set at any positioncan be read from the setting position of the speed change operationtool.

In addition, the output current value of the electromagnetic solenoid ofthe steering HST is so controlled that an output speed ratio of thesteering HST to the traveling HST every when the steering operation toolis set at an arbitrary position is fixed wherever the speed changeoperation tool is set. Accordingly, however speed is set by the speedchange operation tool, a turning radius of the vehicle can be fixed whenthe steering operation tool is set at any position, i.e., at anysteering angle. That is, whether the vehicle travels fast or slowly, adriver can comprehend how much degree the steering operation tool isoperated to correspond to a requested turning radius of the vehicle,thereby facilitating the vehicle to turn properly and improving a senseof turning.

Moreover, since both the ratio of output speed of the traveling HSTevery when the steering operation tool is set at an arbitrary settingposition to the output speed thereof when the vehicle travels straightand the ratio of output speed of the steering HST to output speed of thetraveling HST every when the steering operation tool is set at anarbitrary setting position are fixed wherever the speed change operationtool is set, such an effect can be also obtained that theabove-mentioned steering angle for vanishing the speed of the drive axleon inside in turning when the brake turn begins is fixed.

However, while the output speed of the steering HST is controlled in theabove-mentioned manner, the steering operation tool must be manipulatedto a constant degree for obtaining the same turning radius whether thevehicle travels fast or slowly, thereby giving such an impression thatit is hard to turn the steering wheel when the vehicle travels slowly.Then, according to the present invention, while the setting position ofthe speed change operation tool is within a range for lower speed than acertain value, the output speed ratio of the steering HST to thetraveling HST every when the steering operation tool is set at anarbitrary setting position is larger than the above-mentioned fixedvalue. Therefore, a turning radius corresponding to any fixed steeringangle when the vehicle travels slowly is smaller than that when thevehicle travels fast, thereby improving a response of turning of thevehicle to the steering operation so as to ease the steering operationwhen the vehicle travels at low speed. Incidentally, when the speedchange operation tool is set in another range for higher speed than thecertain value, the traveling HST and the steering HST are controlled intheir output speeds so that the turning radius at every arbitrarysteering angle and the steering angle at start of the brake turn arefixed regardless of variation of traveling speed.

A second object of the present invention is to provide a vehicle whichcontrols the respective output speeds of the traveling HST and thesteering HST with controlling of the electromagnetic solenoids based onpositional detection of the speed change operation tool and the steeringoperation tool, wherein, even if the speed change operation tool ismanipulated quickly, a sudden change of the output speed is avoided soas to ensure safety and comfort.

To attain the second object, according to the present invention, anupper limit is provided to displacement speed of output current value ofthe electromagnetic solenoid for controlling output speed of thetraveling HST. If the displacement speed of the output current valueexceeds the upper limit, the output current value is displaced at theupper limit speed.

A third object of the present invention is to provide a control systemfor automatically neutralizing the traveling HST and the steering HSTonly according to braking operation without operation of the speedchange operation tool and the steering operation tool, thereby saving adriver's labor for manipulation. It is also to provide a vehicle simplyand compactly equipped with such a control system.

To attain the third object, according to the present invention, a brakefor braking both the drive axles is disposed in a transmission system toboth the drive axles. Also, a brake operation tool such as a foot pedalis provided for operating the brake. If a stroke of the brake operationtool reaches a predetermined neutral position, the output current valuesof the electromagnetic solenoids are controlled so as to vanish theoutput speeds of the traveling HST and the steering HST. Therefore,during braking operation, wearing of the drive axles is reduced. Also,manipulation labors can be reduced because both the HSTs isautomatically neutralized for avoiding a sudden start of the vehicle onreleasing the brake. Furthermore, during the brake actuation, the speedchange operation tool and the steering operation tool are maintained attheir state before braking so as to re-create the traveling speed andthe turning angle of the vehicle before braking only by releasing thebraking.

For a detailed configuration of a control system for automaticallyneutralizing the traveling and steering HSTs according to the brakingoperation, a switch is provided so as to change when the brake operationtool is operated at a stroke to the above-mentioned neutral settingposition. The change of the switch is detected with an electric signalso that the output currents of the electromagnetic solenoids arecontrolled so as to vanish the output speeds of the traveling HST andthe steering HST. By such a simple and compact configuration, theabove-mentioned neutralization of both the HSTs when braking can beobtained.

In this configuration, the displacement speeds of output currents of theelectromagnetic solenoids for restoring the output speed of thetraveling HST and the steering HST when the stroke of the brakeoperation tool is reduced across the neutral setting position arerestricted under fixed values so as to prevent the traveling speed andturning angle of the vehicle from being restored suddenly according tobrake-releasing, thereby ensuring safety.

The neutral setting position is set on a stroke position of the brakeoperation tool corresponding to a stroke smaller than that to a brakesetting position for braking actuation so as to make such a doublefunction that the braking operation is performed after neutralizingoperation of both the HSTs. Thus, since outputs of both the HSTs arereduced to some degree or vanished when the brake acts, wearing of thedrive axles and the brake can be reduced and a reasonable brakingprocess from deceleration to braking can be obtained.

In this braking configuration, by restricting the displacement speed ofoutput currents of the electromagnetic solenoids for vanishing theoutput speeds of the traveling HST and the steering HST when the brakeoperation tool reaches the neutral setting position, it occurs certainlythat the vehicle is gradually decelerated and then braked.

A fourth object of the present invention is to provide a vehicle whosetraveling HST and steering HST are electrically controlled in theiroutputs in the above way, wherein the vehicle is safe from unexpectedsudden start when the engine starts.

To attain the fourth object, according to the present invention, enginestarting becomes possible when the brake operation tool is operated atthe stroke to the braking setting position and the neutral location ofthe speed change operation tool is recognized. Accordingly, when thebrake acts but the traveling HST is not neutral, the engine does notstart, thereby avoiding unexpected sudden start of the vehicle when theengine starts.

These, other and further objects, features and advantages will appearmore fully in the following description on the basis of accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS/FIGURES

FIG. 1 is an entire side view of a crawler tractor serving as anembodiment of a vehicle according to the present invention.

FIG. 2 is a diagram of a traveling and steering drive system of thecrawler tractor from an engine to left and right driving sprockets.

FIG. 3 is a diagram of a hydraulic circuit of the traveling HST and thesteering HST in the crawler tractor using electromagnetic proportionalvalves.

FIG. 4 is a diagram of an electric control system for theelectromagnetic proportional valves.

FIG. 5 illustrates respective graphs of a real vehicle-center speed Vc,a real turning-inside speed Vi, a real turning-outside speed Vo and asteering output speed Vs, in relation to a steering angle θ when thespeed of the vehicle traveling straight is V.

FIG. 6 illustrates respective graphs of a standard vehicle-center speedVc1 and an additional vehicle-center speed Vc2 for realizing the realvehicle-center speed Vc, in relation to steering angle θ when the speedof the vehicle traveling straight is V.

FIG. 7 is a graph of a driving coefficient K for establishing steeringoutput speed Vs in relation to a stroke H of a main speed change lever.

FIG. 8 illustrates respective graphs of an output rotary speed TN of thetraveling HST and an output rotary speed SN of the steering HST, inrelation to a current I of the solenoid.

FIG. 9 is a graph of a detected voltage TV of an angle sensor for themain speed change lever in relation to stroke H of the main speed changelever.

FIG. 10 is a graph of a detected voltage SV of an angle sensor for asteering wheel in relation to steering angle θ of the steering wheel.

FIG. 11 illustrates respective graphs of output current TI_(F) of aforward-traveling solenoid and output current TI_(R) of abackward-traveling solenoid when the vehicle travels straight, inrelation to detected voltage TV.

FIG. 12 illustrates respective graphs of output current SI_(R) of aright-turning solenoid and output current SI_(L) of a left-turningsolenoid when the vehicle travels straight, in relation to detectedvoltage TV.

FIG. 13 is a basic flowchart for calculating output currents TI_(FC),TI_(RC), SI_(R) and SI_(L) according to voltages TV and SV.

FIG. 14 is a flowchart of a program R1 for calculating output currentsTI_(FC) and TI_(RC) for forward and backward traveling of the vehicleaccording to voltages TV and SV.

FIG. 15 is a flowchart of a program R2 for calculating output currentsSI_(R) and SI_(L) for right and left turning of the vehicle according tovoltages TV and SV.

FIG. 16 illustrates respective graphs of voltage TV of the sensor fordetecting the position of the main speed change lever and current TI ofthe solenoid for the traveling HST, sharing a common time axis t, on theassumption that an upper limit of the displacement speed of current TIis established.

FIG. 17 is a side view of a linkage from a brake pedal to a brakedevice.

FIG. 18 is a side view of a principal portion showing a positionalrelation between the brake pedal and a brake pedal switch.

FIG. 19 is a controlling flowchart for vanishing the output speeds ofboth the HSTs by changing the brake pedal switch during depression ofthe brake pedal.

FIG. 20 is a controlling flowchart for vanishing the output speeds ofboth the HSTs by detection of a brake pedal position sensor duringdepression of the brake pedal.

FIG. 21 is a controlling flowchart for making an engine impossible tostart after the comprehension of both braking actuation and neutralityof the traveling HST.

BEST MODE FOR CARRYING OUT THE INVENTION

First of all, description will be given of a general structure of acrawler tractor serving as an embodiment of a crawler traveling vehicleaccording to the present invention.

As shown in FIG. 1, a pair of crawler traveling devices 1 are disposedat left and right of the tractor body. An engine 3 is disposed at afront portion of the tractor body and covered with a hood 4.

A cabin 9 is erected behind hood 4. A dashboard 50 is erected at a frontportion in cabin 9, and provided thereon with a steering wheel 7 and abrake pedal 51. A main speed change lever 77 for switching the travelingdirection of the tractor between forward and backward and for steplesslychanging the traveling speed of the tractor, a sub speed change switch76 shown in FIG. 4, and so on are disposed adjacently to a driver's seat8 in cabin 9.

Each of right and left crawler traveling devices 1 comprises a crawlerframe 2 supported below each of the right and left portions of thevehicle body, a driving sprocket 11 supported on a front end portion ofcrawler frame 2, an idler 12 supported on a rear end portion of crawlerframe 2, a plurality of rollers 13 supported on crawler frame 2 betweendriving sprocket 11 and idler 12, and a crawler belt 14 looped overdriving sprocket 11, idler 12, and the plurality of rollers 13.

As shown in FIGS. 1 and 2, a traveling HST 110 serving as a mainspeed-changing device is disposed behind engine 3. Behind traveling HST110 is disposed a transmission casing 5 containing a two-stage (in thisembodiment) sub speed-changing gear mechanism. The total number ofstages provided by a sub speed change system according to thisembodiment is three. In this regard, by manipulating a sub speed changelever (not shown) in cabin 8, a sub speed change clutch 5 a is changedso as to put the sub speed-changing gear mechanism into either alow-speed gear stage or a high-speed gear stage. Furthermore, while thesub speed change lever is set at the low-speed gear stage position, subspeed change switch 76 can be switched between a first low-speed stageand a second low-speed stage. Sub speed change switch 76 changes thetilt setting of a movable swash plate 111 a of a hydraulic pump 111 intraveling HST 110.

A differential unit housing 131 is disposed on the front portion of thevehicle body so as to contain a differential unit 132. A pair of finalspeed-reduction gear boxes 133 are disposed on respective right and leftsides of housing 131 so as to contain respective final speed-reductiongears 134. Each of driving sprockets 11 is disposed on the outside ofeach gear box 133.

According to this embodiment, a pair of right and left planetary gearmechanisms constitute differential unit 132. In this regard, on each ofright and left ends of a lateral sun gear shaft 81 are fixedly providedan input sun gear 32, and freely rotatably provided a carrier 83. On theother hand, a driving axle 86 interlocking with final speed-reductiongear 134 in each gear box 133 is disposed coaxially to sun gear shaft81. In each gear box 133, an output sun gear 85 is fixed on an end ofdriving axle 86. Planet gears 84 are pivoted on each carrier 83 so as tomesh with both input sun gear 82 and output sun gear 85.

FIG. 2 illustrates only the planetary gear mechanism, gear box 133,final speed-reduction gear 134 and driving sprocket 11 on the left sideof differential unit 132, and omits those on the right side ofdifferential unit 132.

In housing 131 is disposed a traveling-driving input shaft 130, whichinterlocks with sun gear shaft 81 through bevel gears and projectsrearward from housing 131. A transmission shaft 72 is interposed throughuniversal joints between a transmission output shaft 6 projectingforward from casing 5 and shaft 130. In this way, power of engine 3transmitted through traveling HST 110 serving as the main speed-changingdevice and the sub speed-changing gear mechanism in transmission casing5 is inputted into input sun gear 82 through transmission shaft 72.

On the other hand, a gear casing 80 and a steering HST 120 are disposedin front of engine 3 so that power of engine 3 is inputted into a gearmechanism in gear casing 80 and a hydraulic pump 121 of steering HST120. In steering HST 120, a hydraulic motor 122 is fluidly connected tohydraulic pump 121. A steering output shaft 122 a of hydraulic motor 122projects into housing 131, and a bevel gear mechanism 135 divides therotation of steering output shaft 122 a into mutually opposite two-flowrotations. The two-flow rotations are inputted into respective carriers83 in right and left planetary gear mechanisms of differential unit 132through gears. That is, each of right and left carriers 83 serves as asection for inputting the output of steering HST 120.

In this way, in each of the right and left planetary gear mechanisms ofdifferential unit 132, the force of input sun gear 82 created by theoutput of traveling HST 110 and the force of carrier 83 created by theoutput of steering HST 120 result in that planet gears 84 revolve aroundsun gear 81 and rotate about their own axes. The revolution and rotationof planet gears 84 is transmitted to output sun gear 85 so as to rotatedriving axle 86, thereby being finally transmitted to each drivingsprocket 11.

By the output force of steering HST 120, mutually opposite rotary forcesare applied onto respective right and left carries 83 so that thedriving force of one carrier 83 is added to the rotary of correspondinginput sun gear 82 and that of the other carrier 83 is subtracted fromthe rotary of corresponding input sun gear 82. Consequently, one ofright and left driving axles 86 is accelerated, and the otherdecelerated, thereby turning the vehicle.

Description will now be given of a structure and a control system oftraveling HST 110 and steering HST 120 in accordance with FIGS. 2 to 4.

Traveling HST 110 is constituted by a variable displacement typetraveling hydraulic pump 111 and a traveling hydraulic motor 112 fluidlyconnected to each other. Traveling hydraulic pump 111 is driven by powerof engine 3 so as to send pressure oil to traveling hydraulic motor 112while the flow direction and volume of the pressure oil correspond tothe controlled position of a movable swash plate 111 a of travelinghydraulic pump 111, thereby controlling the direction and speed ofoutput rotation of traveling hydraulic motor 112.

Steering HST 120 is constituted by variable displacement type steeringhydraulic pump 121 and steering hydraulic motor 122 fluidly connected toeach other. Steering hydraulic pump 121 is driven by power of engine 3so as to send pressure oil to steering hydraulic motor 122 while theflow direction and volume of the pressure oil correspond to thecontrolled position of a movable swash plate 121 a of steering hydraulicpump 121, thereby controlling the direction and speed of output rotationof steering hydraulic motor 122.

According to this embodiment, hydraulic servomechanisms are provided forpositional controlling of respective movable swash plates 111 a and 121a. A traveling electromagnetic proportional valve 61 and a steeringelectromagnetic proportional valve 62 are controlled in locationcorrespondingly to respective output currents of electromagneticsolenoids so as to control the respective hydraulic servomechanismshydraulically.

As shown in FIG. 4, valve 61 for controlling the position of swash plate111 a is provided with two solenoids 61 a and 61 b, which are providedfor forward traveling and backward traveling of the vehiclerespectively. Solenoids 61 a and 61 b are connected to an outputinterface (D/A converter) 141 b of an HST controller 141 for controllingvalve 61 and a later-discussed electromagnetic proportional valve 62. Anangle sensor 78 for detecting the position of main speed change lever 77is connected to an input interface (A/D converter) 141 a of controller141.

When main speed change lever 77 is manipulated, the manipulateddirection and angle thereof from its neutral position are detected withangle sensor 78. On the basis of this detection, controller 141 controlsso that current value corresponding to the detected manipulated anglethereof flows to one of two solenoids 61 a and 61 b, thereby locatemovable swash plate 111 a at a position corresponding to the manipulatedposition of lever 77.

Additionally, sub speed change switch 76 is connected to input interface(A/D converter) 141 a of HST controller 141 so that switch 76 can beswitched between the two positions, i.e., the first low-speed positionand the second low-speed position while the sub speed change lever isset in the low-speed gear stage. Controller 141 predetermines therespective most tilt angles of swash plate 111 a for forward travelingand backward traveling corresponding to each stage of switch 76 and aratio of tilt angle of swash plate 111 a to the shifted angle of mainspeed change lever 77 (i.e., the displacement value of angle sensor 78)within the range bounded by the above-mentioned most tilt angles. Ifswitch 76 is set in the first low-speed stage, each of the most tiltangles of swash plate 111 a (i.e., the most current of each of solenoids61 a and 61 b), which corresponds to a full stroke position of lever 77(i.e., each of the maximum forward-traveling speed position and themaximum backward-traveling speed position thereof) is small, and ifswitch 76 is set in the second low-speed stage, each of the most tiltangles of swash plate 111 a is large. In each of the two cases, the tiltrange of swash plate 111 a is shared by tilt angles of swash plate 111 a(i.e., currents of solenoids 61 a and 61 b) in relation to the shiftedangle of lever 77.

Accordingly, when the first low-speed stage is set, the ratio of tilt ofswash plate 111 a to the shift of lever 77 is small so as to facilitatethe fine adjustment of traveling speed. When the second low-speed stageis set, the ratio of tilt of swash plate 111 a becomes larger. If aconsiderably great acceleration is requested, the second low-speed stagemay be set so as to obtain a considerably great variation of speed inrelation to the shift of lever 77.

Steering electromagnetic proportional valve 62 for controlling theposition of movable swash plate 121 a is provided with two solenoids 62a and 62 b, which are provided for right turning and left turning of thevehicle respectively. Solenoids 62 a and 62 b are connected to outputinterface (D/A converter) 141 b of controller 141. An angle sensor 79for detecting the manipulated angle of steering wheel 7 is connected toinput interface (A/D converter) 141 a of controller 141.

When steering wheel 7 is rotated, the rotated direction and anglethereof from its neutral position (a straight traveling settingposition) are detected with angle sensor 79. On the basis of thisdetection, controller 141 controls so that current value correspondingto the detected rotated angle thereof flows to one of two solenoids 62 aand 62 b, thereby locate swash plate 121 a at a position correspondingto the rotated direction and angle of steering wheel 7.

Furthermore, since it is set that the traveling speed variescorrespondingly to the steering angle as discussed later (referring to alater-discussed real vehicle-center speed Vc), movable swash plate 111a, i.e., solenoids 61 a and 61 b are also controlled on the basis ofdetection by angle sensor 79. On the other hand, since the output ofsteering HST 120 varies correspondingly to the setting speed of mainspeed change lever 77 (referring to a later-discussed steering outputspeed Vs), movable swash plate 121 a, i.e., solenoids 62 a and 62 b arealso controlled on the basis of detection by angle sensor 78.

Moreover, brake pedal sensor 31 for detecting the depression of brakepedal 51 and a brake pedal switch 32, which is turned on when brakepedal 51 is depressed for braking, are connected to the D/A converter ofcontroller 141 so that the output rotary speeds of both hydraulic motors112 and 122 are changed according to the depression degree of brakepedal 51.

Description will now be given of a speed control when the vehicle turns.Hereinafter, “speed” is referred to as a value decided by the settingposition of main speed change lever 77 on the assumption that the enginerotary speed (or a throttle of the engine) is kept constant.

In FIG. 5, graphs in relation to a steering angle θ, which is arotational angle θ of steering wheel 7 rotated either clockwise orcounterclockwise from the straight traveling setting position, are areal vehicle-center speed Vc, which is a real speed of the lateralmiddle portion of the vehicle, a real outside-turning speed Vo, which isa real speed of crawler traveling device 1 on outside of the vehicle inturning, a real inside-turning speed Vi, which is a real speed ofcrawler traveling device 1 on inside of the vehicle in turning, and asteering output speed Vs, which is an added speed of crawler travelingdevice 1 on outside of the vehicle in turning by driving of steering HST120. The sub speed stage may be any of the high-speed stage, the firstlow-speed stage, and the second low-speed stage.

A range of steering angle θ which is equal to or less than θ₁ (0<θ≦θ₁)is provided for a play of steering wheel 7. While steering wheel 7 isrotated in this range, steering HST 120 is not driven (that is, steeringoutput speed Vs is maintained to zero) so that speeds Vc, Vo and Vi atrespective portions of the vehicle are maintained to astraight-traveling speed V.

If steering angle θ exceeds the play-limit angle θ₁, steering HST 120carries out even deceleration and acceleration of steering output speedVs in the respective turning-inside and turning-outside drivingsprockets 11 in substantially proportion to the increase of steeringangle θ, thereby creating real turning-inside speed Vi and realturning-outside speed Vo (see Formula I).Vo=Vc+Vs, Vi=Vc−Vs  Formula I

Real vehicle-center speed Vc, which is an average of realturning-outside speed Vo and real turning-inside speed Vi, issubstantially created by the output of traveling HST 110 correspondingto any steering angle θ because the deceleration of turning-insidedriving sprocket 11 and the acceleration of turning-inside drivingsprocket 11, that are caused by steering HST 120, are equal to eachother. That is, by the setting of HST controller 141, the output rotaryspeed of steering HST 120 is essentially controlled so as to decreaseaccording to increase of steering angle θ(≧θ₁) so that realvehicle-center speed Vc also decreases according to increase of steeringangle θ(≧θ₁).

When it is assumed that the output rotary speed of traveling HST 110decreases in proportion to steering angle θ(≧θ₁), the vehicle-centerspeed is expressed with a rectilinear graph slanting upwardly rightwardshown in FIG. 6. This will be called a standard vehicle-center speedVc1.

For example, it is assumed that standard vehicle-center speed Vc1 inrelation to steering angle θ(≧θ₁) becomes p₁ (0<p₁<1) timesstraight-traveling speed V when steering angle θ reaches the maximumangle θ_(MAX) . Standard vehicle-center speed Vc1 is calculated withFormula II. ${Formula}\quad{{II}:\begin{matrix}{{Vc} = {V - {\left( {V - {p_{1}*V}} \right)*{\left( {\theta - \theta_{1}} \right)/\left( {\theta_{MAX} - \theta_{1}} \right)}}}} \\{= {V*{\left\{ {1 - {\left( {1 - p_{1}} \right)*\left( {\theta - \theta_{1}} \right)}} \right\}/\left( {\theta_{MAX} - \theta_{1}} \right)}}} \\{\theta_{1}\quad \leqq \quad\theta\quad \leqq \quad\theta_{MAX}}\end{matrix}}$

According to this variation of standard vehicle-center speed Vc1 inrelation to steering angle θ, the speed reduction rate of vehicle-centerspeed Vc is fixed whether steering angle θ is small or large. However,even if the speed of the vehicle center is reduced appropriately in thestate where steering angle θ is large (e.g., the maximum angle θ_(MAX)),the vehicle-center speed while steering angle θ being small is desiredto be almost the same with that when the vehicle travels straight.Furthermore, it is desirable that the vehicle-center speed is graduallyreduced as steering angle θ increased to some degree, and the rate ofdeceleration thereof becomes large so as to enable the vehicle to do asmall turn when steering angle θ becomes close to maximum angle θ_(MAX).

Then, as shown in FIG. 6, an additional vehicle-center speed Vc2 isestablished as a correction value to be added to standard vehicle-centerspeed Vc1, and real vehicle-center speed Vc is calculated with FormulaIII.Vc=Vc 1+Vc 2  Formula III

Thus, such a graph of real vehicle-center speed Vc as shown in FIG. 5 isobtained.

It is made into Vc2=0 at the time of 0≦θ≦θ₁. At the time of θ≧θ₁, ifstraight-traveling vehicle V is made to be one and steering angle (θ−θ₁)serves as a variable, Vc2 may be calculated with a quadratic functionf₁(θ−θ₁) so that it becomes the maximum value Vc2 _(MAX) at the timeθ=θ₁+(θ_(MAX)+θ₁)/2=(θ_(MAX)−θ₁)/2, and it becomes zero at the time ofθ=θ₁ and at the time of θ=θ_(MAX). Furthermore, maximum value Vc2 _(MAX)is made to vary in proportion to straight-traveling speed V. Then, Vc2is calculated with Formula IV.Vc 2=V*f ₁(θ−θ₁) Formula IV:

For establishing function f₁(θ−θ₁), it is made that Vc2≈V−Vc1 at thetime of θ₁ ≦θ<θ₁+(θ_(MAX)—θ₁)/2=(θ_(MAX)+θ₁)/2, whereby Vc2 increases ata tempo that is substantially equal to the decreasing tempo of Vc1 thatdecreases in proportion to increase of steering angle θ, thereby almostmaintaining Vc at straight-traveling speed V. In course of time, theincreasing tempo of Vc2 in relation to steering angle θ is reduced.After steering angle θ becomes larger than (θ_(MAX)+θ₁)/2, Vc2 isacceleratedly reduced according to increase of steering angle θ.Therefore, Vc is gradually reduced according to increase of steeringangle θ, and it is reduced in an increased tempo as steering angle θbecomes close to maximum angle θ_(MAX). Finally, it is made that Vc=Vc1when steering angle θ reaches maximum angle θ_(MAX).

Real vehicle-center speed Vc calculated in this way is constant at everyarbitrary steering angle θ in relation to straight-traveling speed Vhowever speed V varies. Thus, in the state where main speed change lever77 is set to any of various speed-setting positions, the vehicle-centerspeed when steering wheel 7 is rotated to any degree can be read fromthe set speed. If steering wheel 7 is rotated to a certain angleθ_(FIX), for example, real vehicle-center speed Vc can be read to bealways V/2, e.g., 5 km/s when straight-traveling speed V is 10 km/s, or10 km/s when straight-traveling speed V is 20 km/s.

Description will be given of control of output of steering HST 120 inrelation to steering angle θ, i.e., control of steering output speed Vs.As shown in FIG. 5, it is kept that Vs=0 at the time of θ<θ₁. In thecase of θ₁≦θ≦θ_(MAX) , first of all, steering output speed V_(SMAX) atmaximum steering angle θ_(MAX) is calculated. Vs is made to increaserectilinearly to the calculated maximum V_(SMAX) according to increaseof steering angle θ.

As shown in FIG. 5, for example, it is assumed that maximum steeringoutput speed V_(SMAX) is p₂ times straight-traveling speed V andrectilinearly increases from 0 to V*p₂ while steering angle θ increasesfrom θ₁ to θ_(MAX). That is, Vs is calculated with Formula VI.Vs=V* p ₂*(θ−θ₁)/(θ_(MAX)−θ₁) θ₁≦θ≦θ_(MAX)  Formula VI

Then, it is assumed that steering angle θ becomes θ₂ when realturning-inside speed Vi reaches 0, i.e., at the time of start of thebrake turn. If this assumption is made true regardless ofstraight-traveling speed V, Formula VII is materialized with FormulasII, III and IV and the function of Vc2.V−(V−p ₁ *V)*(θ₂−θ₁)/(θ_(MAX)−θ₁)+V*f ₁(θ₂−θ₁)=V*p₂*(θ₂−θ₁)/(θ_(MAX)−θ₁)  Formula VII:

Accordingly, p₂ can be calculated with Formula VII′.p ₂={1+f ₁(θ₂−θ₁)}*(θ_(MAX)−θ₁)/(θ₂−θ₁)−1 +p ₁  Formula VII′

Value p₂ is decided so as to materialize VII′. V_(SMAX) is establishedon the basis of decided p₂ so as to fix steering angle θ on thebeginning of brake turn to θ₂.

Besides, as mentioned above, Vc1, Vc2 and Vs vary in proportion tostraight-traveling speed V so that real vehicle-center speed Vc, realturning-inside speed Vi and real vehicle-outside speed Vo vary inproportion to straight-traveling speed V when each of allowed steeringangles θ(≦θ₁) is set. That is, as mentioned above, the ratio of outputspeed of traveling HST 110 to straight-traveling speed V, i.e., Vc/V,and the ratio of output speed of steering HST 120 to that of travelingHST 110, i.e., Vs/Vc are constant at every steering angle θ regardlessof variation of straight-traveling speed V, thereby fixing the turningradius of the vehicle regardless of straight-traveling speed V.

However, steering output speed Vs decided in this way becomes extremelysmall when straight-traveling speed V is small, that is, when main speedchange lever 77 is set at a low-speed position, thereby worsening thereaction of the vehicle to rotation of steering wheel 7. Then, whenlever 77 is set at a low-speed position, maximum steering output speedV_(SMAX) is made larger than V*p₂ so as to enlarge steering output speedVs at every steering angle θ. Therefore, it is made that V_(SMAX)=K*V*p₂while K is a driving coefficient of steering HST 120 established asfollows:

First, a stroke of main speed lever 77 from its neutral position withineach of its respective ranges for forward traveling and backwardtraveling while it is set in any of the sub speed stages is referred toas H. The maximum stroke thereof is H_(MAX), which is the same whetherthe vehicle travels forward or backward. Furthermore, there ispredetermined a certain stroke H₂, which is the same whether the vehicletravels forward or backward. It is made that K=1 at the time ofH₂≦H≦H_(MAX), and it is made that K=f₂(H)>1 at the time of 0≦H≦H₂.

This function f₂(H) may be a quadratic function, for example, such thatdriving coefficient K becomes maximum K_(MAX) at the time of H=0, and itbecomes 1 at the time of H=H₂.

FIG. 7 illustrates a correlation diagram of the driving coefficient ofsteering HST 120 in relation to stroke H of main speed change lever 77from its neutral position in the above-mentioned way.

Vs is calculated with driving efficient K through Formula VI′.

 Vs=K*V*p ₂*(θ−θ₁) /(θ_(MAX)−θ₁)θ₁≦θ≦θ_(MAX)  Formula VI′

Referring to some effects of such calculation of steering output speedVs, when stroke H of lever 77 is larger than H₂ (for setting a higherspeed), as mentioned above, the turning radius of the vehicle at anysteering angle and the steering angle for starting of the brake turn arefixed regardless of the speed of the vehicle in straight-traveling. Whenstroke H of main speed change lever 77 is smaller than H₂ for setting alower speed, the smaller stroke H is, the smaller the turning radius ofthe vehicle becomes and the earlier the brake turn begins (it means thatsteering angle θ on the beginning of brake turn becomes smaller than θ₂)in comparison with the case where main speed change lever 77 is set fora higher speed. However, paradoxically speaking, this makes the sense inturning of the vehicle at low speed better.

Controller 141 evaluates values Vc1, Vc2 and Vs with Formulas II, III,IV, VI and VII for calculating real vehicle-center speed Vc, realturning-inside speed Vi and real turning-outside speed Vo at everyarbitrary steering angle θ on the basis of detection values of anglesensor 78 for detecting the stroke position H of main speed change lever77 and angle sensor 79 for detecting the rotational angle θ of steeringwheel 7. Furthermore, controller 141 controls traveling electromagneticproportional valve 61 and steering electromagnetic proportional valve 62so as to realize real vehicle-center speed Vc, real turning-inside speedVi and real turning-outside speed Vo every when any steering angle iscalculated in this way.

In fact, there is needed a process for calculating a current value forenergizing the solenoid of each of electromagnetic proportional valves61 and 62 on the basis of voltages detected by angle sensors 78 and 79.This process will be described in accordance with FIGS. 8 to 15.

Electromagnetic solenoids 61 a, 61 b, 62 a and 62 b are standardized soas to have the same sized effective ranges for their respective outputcurrents I from 0 (ampere) to I_(MAX) (amperes).

As shown in FIG. 8, an output rotary speed TN (rpm) of traveling HST 110is essentially increased from 0 to TN_(MAX) in proportion to increase ofeither a forward-traveling current TI_(F) serving as an output currentof forward-traveling electromagnetic solenoid 61 a or abackward-traveling current TI_(R) serving as an output current ofbackward-traveling electromagnetic solenoid 61 b. A traveling current TIis given to these output currents TI_(F) and TI_(R) as a generic name.It results in TN=k₁*TI, while k₁ is a proportionality factor.

Besides, an output rotary speed SN (rpm) of steering HST 120 isessentially increased from 0 to SN_(MAX) in proportion to increase ofeither a right-turning current SI_(R) serving as an output current ofright-turning electromagnetic solenoid 62 a or a left-turning currentSI_(L) serving as an output current of left-turning electromagneticsolenoid 62 b. A steering current SI is given to these output currentsSI_(R) and SI_(L) as a generic name. It results in SN=k₂*SI, while k₂ isa proportionality factor.

A range where current value I is close to 0 so as to maintain both TNand SN to 0, and a range where current value I is close to maximumI_(MAX) so as to keep TN=TN_(MAX) and SN=SN_(MAX) may be neglected

The voltage detected by angle sensor 78 expressing the position of mainspeed change lever 77 is named TV (volts), and the voltage detected byangle sensor 79 expressing the position of steering wheel 7 is named SV(volts).

FIG. 9 illustrates a correlation between stroke H of main speed changelever 77 from its neutral position and detected voltage TV of anglesensor 78. Stroke H of main speed change lever 77 at the neutralposition is 0. The stroke thereof in a range for forward traveling ofthe vehicle is named a forward-traveling stroke FH, and its maximumstroke is named FH_(MAX). The stroke thereof in a range for backwardtraveling of the vehicle is named a backward-traveling stroke RH, andits maximum stroke is named RH_(MAX). While stroke H varies from maximumbackward-traveling stroke RH_(MAX) through the neutral stroke 0 tomaximum forward-traveling stroke RH_(MAX), detected voltage TVproportionally increases from minimum value TV_(MIN) to maximum valueTV_(MAX) through an intermediate value TV_(N) corresponding to H=0.

FIG. 10 illustrates a correlation between steering angle θ and detectioncurrent value SV of angle sensor 79. Steering angle θat the neutralposition (the straight-traveling position) of steering wheel 7 is 0.Steering angle θ for right-turning of the vehicle is named right-turningangle Rθ(≧0), and steering angle θ for left-turning of the vehicle isnamed left-turning angle Lθ(≧0). While steering wheel 7 is rotated frommaximum left-turning angle Lθ_(MAX) through the neutral position (whereθ=0) to maximum right-turning angle Rθ_(MAX); current value SVproportionally increases from minimum value SV_(MIN) to maximum valueSV_(MAX) through an intermediate value SV_(N) corresponding to theneutral position.

FIG. 11 illustrates respective graphs of forward-traveling current valueTI_(F) and backward-traveling current value TI_(R) in relation todetected voltage TV when the vehicle travels straight, that is, the timeof 0≦θ≦θ₁. A stroke range of main speed change lever 77 fromback-traveling stroke RH₁ to forward-traveling stroke FH₁ through theneutral position serves as a neutral range (for making V=0). Detectedvoltage TV of angle sensor 79 corresponding to RH₁ is named TV_(R1), andthat corresponding to FH₁ is named TV_(F1). In the case ofTV_(R1)<TV<TV_(F1), current values TI_(F) and TI_(R) are maintained in0.

Forward-traveling current TI_(F) proportionally increases from 0 tomaximum value I_(TMAX) while detected voltage TV varies from TV_(F1) tomaximum value TV_(MAX) corresponding to the maximum forward-travelingspeed. Backward-traveling current TI_(R) proportionally increases from 0to maximum value I_(TMAX) while detected voltage TV varies from TV_(R1)to minimum value TV_(MIN) corresponding to the maximumbackward-traveling speed.

That is, currents TI_(F) and TI_(R) for straight-traveling of thevehicle is calculated through Formula VIII with detected voltage TVwhile a (>0) serves as a fixed proportionality factor.At the time of TV≦TV _(R1) , TI _(R) −a*(TV−TV _(R1))TI _(F)=0At the time of TV _(R1) <TV<TV _(F1) , TI _(F) =TI _(R)=0At the time of TV≧TV _(F1) , TI _(F) =a*(TV−TV _(F1))TI_(R)=0  Formula VIII

Accordingly, in connecting with above-mentioned Formulas II to IV forcalculating real vehicle-center speed Vc, steering angle θ (Rθ forright-turning or Lθ for left-turning) can be calculated with voltage SVon the basis of the graph shown in FIG. 10. Furthermore, either TI_(F)or TI_(R) replaces straight-traveling speed V and is replaced with acurrent value calculated through Formula VIII on the basis of detectedvoltage TV so that a standard forward-traveling current TI_(FC1)replaces Vc1 in forward traveling of the vehicle, a standardbackward-traveling current TI_(RC1) replaces Vc1 in backward travelingof the vehicle, an additional forward-traveling current TI_(FC2)replaces Vc2 in forward-traveling of the vehicle, and an additionalbackward-traveling current TI_(RC2) replaces Vc2 in backward-travelingof the vehicle. Therefore, each of forward-traveling current TI_(FC) forcreating real vehicle-center speed Vc every steering angle θ inforward-traveling of the vehicle and backward-traveling current TI_(RC)for creating real vehicle-center speed Vc every steering angle θ inbackward-traveling of the vehicle can be calculated on the basis ofdetected voltage TV.

Consequently, the graph of standard vehicle-center speed Vc1 in relationto steering angle θ in FIG. 6 may be replaced with a graph of standardforward-or-backward-traveling current value TI_(FC1) or TI_(RC1) inrelation to either steering angle Rθ or Lθ. Also, the graph ofadditional vehicle-center speed Vc2 in relation to steering angle θ inFIG. 6 may be replaced with a graph of additionalforward-or-backward-traveling current value TI_(FC2) or TI_(RC2) inrelation to either steering angle Rθ or Lθ.

In FIG. 11, a voltage TV_(F2) corresponds to a stroke H2 of main speedchange lever 77 for forward traveling of the vehicle, named FH₂, whichconcerns the determination of above-mentioned driving coefficient K.Also, a voltage TV_(R2) corresponds to a stroke H2 thereof for backwardtraveling of the vehicle, named RH₂. The graph of driving coefficient Kin relation to stroke H in FIG. 7 may be replaced with a graph ofdriving coefficient K either when detected voltage TV increases fromTV_(N) to TV_(MAX) or when detected voltage TV decreases from TV_(N) toTV_(MIN).

Then, in Formula VI′ on the basis of Formula VII′, Vs and V aresubstituted for SN and TN, respectively, so as to form an equality forcalculating output rotary speed SN of steering HST 120 every steeringangle θ on the basis of output rotary speed TN of traveling HST 110 setfor straight traveling of the vehicle. Furthermore, since it ismaterialized of TN=k₁*TI, output rotary speed TN is substituted fork₁*TI_(F) or k₁*TI_(R). Further, SN is substituted for k₂*SI. Therefore,an equality is materialized so as to calculate current SI on the basisof steering angle θ and detected voltage TV. According to the graph ofFIG. 10, an arbitrary steering angle θ (which is right steering angle Rθor left steering angle Lθ) is substituted for a corresponding voltage SVso as to get a following equality for calculating both right-turningcurrent SI_(R) and left-turning current SI_(L) on the basis of voltageSV.At the time of SV≦SV _(L1) , SI _(R)=0SI _(L) =−K*TI _(F) *b*(SV−SV _(L1))or SI _(L) =−K*TI _(R) *b*(SV−SV _(L1))At the time of SV _(L1) <SV<SV _(R1) , SI _(L) =SI _(R)=0At the time of SV≦SV _(L1) , SI _(L)=0SI _(R) =K*TI _(F) *b*(SV−SV _(L1))or SI _(R) =K*TI _(R) *b*(SV−SV _(L1))  Formula IX

In Formula IX, b(>0) is a fixed proportionality factor which is decidedby Formula VI′ on the basis of Formula VII′ with substitution of Vs fork₂*SI and substitution of V for k₁*TI_(F) or k₁*TI_(R) and according tothe relation of current value to right steering angle Rθ and leftsteering angle Lθ illustrated by FIG. 10.

Furthermore, TI_(F) and TI_(R) are calculated with Formula VIII on thebasis of voltage TV. Consequently, right-turning current SI_(R) andleft-turning current SI_(L) can be decided on the basis of detectedvoltage TV of angle sensor 78 for main speed change lever 77 anddetected voltage SV of angle sensor 79 for steering wheel 7.

According to Formula IX, when main speed change lever 77 is put at onesetting position, there is established a graph for setting right-turningcurrent SI_(R) and left-turning current SI_(L) in relation to detectedvoltage SV, as shown in FIG. 12. K*TI_(F)*b and −K*TI_(F)*b in FormulaIX correspond to the inclinations of the graph. Therefore, theinclination of this graph and maximum current value I_(SMAX) varyaccording to the variation of straight-traveling speed V set by lever 77(i.e., detected voltage TV of angle sensor 78) and according to thevariation of driving coefficient K which becomes variable when lever 77is within a certain setting range (RH₂<H<FH₂).

Description will now be given of processes for electrically controllingrespective solenoids of electromagnetic proportional valves 61 and 62 onthe basis of detection by angle sensors 78 and 79 in accordance withFIGS. 13 to 15, so as to describe the process for controlling theoutputs of traveling HST 110 and steering HST 120 by manipulation ofmain speed change lever 77 and steering wheel 7.

The outputs of valves 61 and 62 are basically controlled through thecontrolling process shown in FIG. 13. In this regard, voltage TV isinputted from angle sensor 78 (step S101), and voltage SV is inputtedfrom angle sensor 79 (step S102). The output of traveling HST 110 iscontrolled through an output current controlling program R1 fortraveling electromagnetic proportional valve 61 (step S103), and theoutput of steering HST 120 is controlled through an output currentcontrolling program R2 for steering electromagnetic proportional valve62 (step S104).

Output current controlling program R1 for traveling electromagneticproportional valve 61 will be described in accordance with FIG. 14.First, inputted voltage TV is applied into Formula VIII. At the time ofTV_(R1)<TV<TV_(F1) (step S201), both forward-traveling current valueTI_(F) and backward-traveling current value TI_(R) are 0 so as to setthe straight-traveling speed to 0 (step S202). At the time of TV≧TV_(F1)(step S203), forward-traveling current value TI_(F) for creating forwardstraight-traveling speed V is calculated through the equality ofTI_(F)=a*(TV−TV_(R1)) on the basis of detected voltage TV (step S204).At this time, backward-traveling current value TI_(R) is 0 (omitted inFIG. 14). And at the time of TV≦TV_(R1) (step S215), backward-travelingcurrent value TI_(R) for creating backward straight-traveling speed V iscalculated through the equality of TI_(R)=a*(TV−TV_(F1)) on the basis ofdetected voltage TV (step S216). At this time, forward-traveling currentvalue TI_(F) is 0 (omitted in FIG. 14).

Whether the vehicle travels forward or backward, if the detection ofvoltage SV results in SV_(R1)<SV<SV_(F1) (step S205 or S217), thevehicle is made to travel straight. That is, either forward-travelingcurrent TI_(FC) or backward-traveling current TI_(RC) for creating realvehicle-center speed Vc is maintained to TI_(F) or TI_(R) for straighttraveling (step S206 or S218).

It is assumed that the vehicle travels forward. If it is materializedthat SV≧SV_(R1) (step S207), the vehicle turns right so that rightsteering angle Rθ is calculated on the basis of the graph of FIG. 9, andθ is substituted for calculated Rθ in Formulas II and IV (step S208). Ifit is materialized that SV≦SV_(L1) (step S209), the vehicle turns leftso that left steering angle Lθ is calculated on the basis of the graphof FIG. 9, and θ is substituted for calculated Lθ in Formulas II and IV(step S210). Furthermore, V=TI_(F), Vc1=TI_(FC1), Vc2=TI_(FC2) andVc=TI_(FC) are materialized (step S211), and they are applied intoFormula II so as to calculate standard vehicle-center current valueTI_(FC1) for creating standard vehicle-center speed Vc1 (step S212),applied into Formula IV so as to calculate additional vehicle-centercurrent value TI_(FC2) for creating additional vehicle-center speed Vc2(step S213), and applied into Formula III so as to calculate realvehicle-center current value TI_(FC) of electromagnetic solenoid 61 afor creating real vehicle-center speed Vc every arbitrary steering anglein forward traveling of the vehicle (step S214).

It is assumed that the vehicle travels backward. If it is materializedthat SV≧SV_(R1) (step S219), the vehicle turns right so that rightsteering angle Rθ is calculated on the basis of the graph of FIG. 9, andθ is substituted for calculated Rθ in Formulas II and IV (step S220). Ifit is materialized that SV≦SV_(L1) (step S221), the vehicle turns leftso that left steering angle Lθ is calculated on the basis of the graphof FIG. 9, and θ is substituted for calculated Lθ in Formulas II and IV(step S222). Furthermore, V=TI_(F), Vc1=TI_(RC1), Vc2=TI_(RC2) andVc=TI_(RC) are materialized (step S223), and they are applied intoFormula II so as to calculate standard vehicle-center current valueTI_(FC1) for creating standard vehicle-center speed Vc1 (step S224),applied into Formula IV so as to calculate additional vehicle-centercurrent value TI_(RC2) for creating additional vehicle-center speed Vc2(step S225), and applied into Formula III so as to calculate realvehicle-center current value TI_(RC) of electromagnetic solenoid 61 bfor creating real vehicle-center speed Vc every arbitrary steering anglein backward traveling of the vehicle (step S226).

Output current controlling program R2 for steering electromagneticproportional valve 62 will now be described in accordance with FIG. 15.

First, inputted voltage TV is measured so as to determine drivingcoefficient K (step S301). At the time of TV_(R2)≦TV≦TV_(F2) (stepS201), voltage TV is substituted for either forward-traveling stroke FRor backward-traveling stroke RH so as to materialize K=f₂(FH) orK=f₂(RH) (step S302). At the time of TV>TV_(F2) or TV<TV_(R2), K=0 ismaterialized (step S303).

Then, it is judged whether detected voltage TV is greater or smallerthan voltage TV_(F1) so as to decide whether main speed change lever 77is set for forward traveling or not (step S304). If it is detected thatTV<TV_(F1), it is further judged whether detected voltage TV is greateror smaller than voltage TV_(R1) so as to decide whether main speedchange lever 77 is set for backward traveling or not (step S305). If itis decided that main speed change lever 77 is not set for backwardtraveling, it is set in neutral so that both forward-traveling andbackward-traveling currents TI_(F) and TI_(R) are set to 0 (step S306).

When it is judged to be set for forward traveling, TI_(F)=a*(TV−TV_(F1))is materialized according to Formula VIII, and current TI_(F)corresponding to forward straight-traveling speed is calculated on thebasis of detected voltage TV (step S307). At this time, TI_(R) si 0(omitted in FIG. 15). When it is judged to be set for backwardtraveling, TI_(R)=−a*(TV−TV_(R1)) is materialized according to FormulaVIII, and current TI_(R) corresponding to backward straight-travelingspeed is calculated on the basis of detected voltage TV (step S313). Atthis time, TI_(F)is 0 (omitted in FIG. 15).

Whether the vehicle travels forward or backward, it is judged whetherdetected voltage SV is greater or smaller than voltage SV_(R1) so as todecide whether steering wheel 7 is set for right turning or not (stepS308 or S314). If it is detected that SV<SV_(R1), it is further judgedwhether detected voltage SV is greater or smaller than voltage SV_(L1)so as to decide whether steering wheel 7 is set for left turning or not(step S309 or S315). If it is decided that steering wheel 7 is not setfor left turning, it is set for straight traveling so that bothright-turning and left-turning currents TI_(F) and TI_(R) are set to 0(step S310 or S316).

Whether the vehicle travels forward or backward, the vehicle turns rightat the time of SV≦SV_(R1) so that right-turning current value SI_(R) iscalculated through Formula IX (step S311 or S317), and left-turningcurrent value SI_(L) is 0 (omitted in FIG. 15). The vehicle turns leftat the time of SV≦SV_(L1) so that left-turning current value SI_(L) iscalculated through Formula IX (step S312 or S318), and right-turningcurrent value SI_(R) is 0 (omitted in FIG. 15).

In this way, corresponding to detections of angle sensor 78 for mainspeed change lever 77 and angle sensor 79 for steering wheel 7, eithercurrent TI_(CF) of solenoid 61a or current TI_(CR) of solenoid 61 b forcreating real vehicle-center speed Vc is calculated through program R1for traveling electromagnetic valve 61, and either current value SI_(R)of solenoid 62 a or current value SI_(L) of solenoid 62 b for creatingsteering output speed Vs is calculated through program R2 for steeringelectromagnetic valve 62. Then, the output of each solenoid iscontrolled so as to create speed Vi of inside crawler traveling device Iin turning and speed Vo of outside crawler traveling device 1 inturning.

The above description has been given of control of electromagneticproportional valves 61 and 62 for controlling respective movable swashplates 111 a and 121 a of both HSTs 110 and 120 during turning of thevehicle while main speed change lever 77 is fixed in location.

Next, description will be given of control of traveling electromagneticproportional valve 61 when main speed change lever 77 is shifted foracceleration or deceleration.

For example, it is assumed that main speed change lever 77 is suddenlyshifted for acceleration so as to increase detected voltage of anglesensor 78 from TV₁ to TV₂ (>TV₁) while the vehicle travels straight.Current TI_(F) of energized solenoid 61 a corresponding to detectedvoltage TV₁ is named TI_(F) ₁, and that corresponding to detectedvoltage TV₂ is named TI_(F2). Essentially, current TI increases fromTI_(F) ₁ to TI_(F2) synchronously to displacement of voltage TV so as toincrease the output of traveling HST 110 at a sitting. However, such asudden increase of output of traveling HST 110 shocks a driver anddamages parts.

Therefore, in connection with a current displacement speed TI′, whichserves as a displacement degree of current value TI of travelingelectromagnetic proportional valve 61 (solenoid 61 a or 61 b) every unitof time to (whether the vehicle travels forward or backward), TI′_(MAX)is predetermined. When current displacement speed TI′ corresponding tothe shift speed of shifted main speed change lever 77, i.e.,displacement speed TV′ of voltage TV is equal to or under TI′_(MAX),output current value TI of traveling electromagnetic proportional valve61 varies synchronously to displacement of voltage TV. If currentdisplacement speed TI′ exceeds TI′_(MAX), output current value TI ofvalve 61 varies at a speed according to current displacement speedTI′_(MAX).

Namely, when the shift speed of lever 77 exceeds the speed correspondingto maximum current displacement speed TI′, current TI increases ordecreases at TI′_(MAX) every time unit so as to approach current valueTI corresponding to voltage TV detected after completion of the shift.

FIG. 16 illustrates variation of voltage TV and current TI sharing thesame axis of time t. Whether the shift is for acceleration ordeceleration, maximum current displacement speed TI′_(MAX) is constantin this embodiment. Alternatively, TI′_(MAX) may be different. It may bealso different depend upon which direction the vehicle travels inforward or backward. Furthermore, it may be fit for the shift acrossranges for forward traveling and backward traveling.

Consequently, even if main speed change lever 77 is manipulated quickly,the output displacement speed of traveling HST 110 can be restrictedunder a certain value so as to avoid sudden acceleration anddeceleration of the vehicle.

Incidentally, maximum current displacement speed TI′_(MAX) may alsoserve as respective current displacement speeds of the solenoids inlater-discussed control of traveling HST 110 and steering HST 120 fortheir neutralization and restoring of output at the time of braking.

Description will be given of braking control according to the presentinvention. Conventionally, braking operation is separate from operationfor neutralizing the HST so that a driver must shift the speed changelever for regulating output of the HST to the neutral position andseparately operate for braking the driving sprockets surely (usually,this braking operation is depression of a pedal). In the case ofslamming on the brake, the driving sprockets are braked while they aredriven because there is no time for returning the speed change lever tothe neutral position. Thus, there arises a friction wearing relevantcomponents.

According to the present invention, the HST-neutralization operation andthe braking operation are performed in a double action by manipulationof only brake pedal 51. Namely, brake pedal 51 is also used as a clutchpedal.

Description will now be given of brake pedal 51, a sensor for detectingthe position of brake pedal 51, and a linkage 40 interposed betweenbrake pedal 51 and brake device 58.

Brake pedal 51 is a foot pedal suspended so as to be rotatable around apedal support shaft 52 disposed laterally in dashboard 50. A spring 59biases brake pedal 51 upward.

Brake device 58 is of a wet multi-disc type, for example. As shown inFIG. 2, within housing 131, brake device 58 is disposed around inputshaft 130 of differential unit 132, which is connected with transmissionshaft 72. Brake device 58 directly brakes shaft 130, thereby brakingboth sprockets 11 of left and right crawler traveling devices 1simultaneously.

Brake device 58 according to this embodiment is switched between abraking mode and a release mode by pushing and pulling shaft 130 in itsaxial direction (i.e., in the longitudinal direction of the vehicle).Shaft 130 can be pulled and pushed within a backlash range of the bevelgears. Brake linkage 40 is interposed between shaft 130 and brake pedal51 so as to push and pull shaft 130, as shown in FIG. 17.

Brake linkage 40 will be described. A first fulcrum shaft 54 is disposedlaterally below shaft 52. An approximately vertical connection rod 53pivotally connects an arm 51 a, which rotates integrally with brakepedal 51, and an arm 54 a, which is rotatable around shaft 54 so as tobe pushed and pulled substantially vertically.

Furthermore, an arm 54 b is disposed so as to be rotatable integrallywith arm 54 a around shaft 54. On the other hand, a second fulcrum shaft56 is disposed laterally above a rear portion of shaft 130 projectingfrom housing 131, and arms 56 a and 56 b are disposed so as to bemutually integrally rotatable around shaft 56. Arms 56 a and 56 b aremutually connected through an approximately longitudinal connection rod55. Arm 56 b and shaft 130 are mutually connected through anapproximately vertical rod 57. Arm 56 a is connected to rod 55 byinserting a pin fixed on arm 56 a into a slot 55 a in an utmost end ofrod 55. A play of brake pedal 51 is established by a slidable range ofthe pin of arm 56 a within slot 55 a.

In this way, brake linkage 40 is so constructed that, by rotating brakepedal 51, rod 53 is pulled or pushed substantially vertically, rod 55substantially longitudinally, and rod 57 is tilted forward or rearward,thereby sliding shaft 130 longitudinally.

Brake device 58 raises its braking force as the downward rotationalangle of brake pedal 51 increases. When brake pedal 51 reaches aposition P2 shown in FIG. 18, which is close to the most depressedposition, brake device 58 brakes left and right sprockets 11approximately perfectly.

As shown in FIG. 17, a brake pedal position sensor 31 is disposedadjacently to brake pedal 51 so as to detect the rotational angle (thedepression degree) of pedal 51. In this embodiment, sensor 31 isdisposed adjacently to an upper end of brake pedal 51. However, sensor31 may be disposed at any position if it attains the prescribed object.

Brake pedal switch 32 is disposed so as to turn on when brake pedal 51is depressed to a fixed position. In the embodiment of FIG. 17, switch32 is located so as to turn on when brake pedal 51 is depressed toposition P2. Alternatively, it may be disposed so as to turn on whenpedal 51 is depressed to a position P1 which is shallower than positionP2.

Switch 32 forcibly brings the output speeds of traveling HST 110 andsteering HST 120 into zero. In the case where each of movable swashplates 111 a and 121 a of HSTs 110 and 120 is connected to both mainspeed change lever 77 and steering wheel 7 through a mechanical linkage,both HSTs 110 and 120 are neutralized so as to necessarily return mainspeed change lever 77 to the neutral position and steering wheel 7 tothe straight-traveling position. However, according to the presentinvention, the manipulated positions of main speed change lever 77 andsteering wheel 7 are electrically detected, and electromagneticproportional valves 61 and 62 in the respective hydraulicservomechanisms are electrically controlled on the basis of the detectedvalues so as to move swash plates 111 a and 121 a. Thus, the positionalrelation of main speed change lever 77 and steering wheel 7 to movableswash plates 111 a and 121 a is not fixed. More specifically, main speedchange lever 77 is not set to the neutral position even if brake pedal51 is rotated so as to neutralize traveling HST 110 forcibly.Consequently, when brake pedal 51, which has been depressed so as toneutralize HST 110, becomes unpressed, the traveling speed just beforebraking is re-created easily because main speed change lever 77 is heldat its position before depression of pedal 51.

Furthermore, since the output of traveling HST 110, i.e., realvehicle-center speed Vc, and the output of steering HST 120, i.e.,steering output speed Vs are controlled every arbitrary steering angleas mentioned above according to the present invention, brake pedal 51,which has been depressed just after steering wheel 7 is set to asteering angle for left or right turning, becomes unpressed so that thevehicle resumes its turning on a turning radius corresponding to thesteering angle which has been set by steering wheel 7.

Description will be given of control of output of the HST on the basisof judge whether switch 32 is set on or off in accordance with FIG. 19.

It is presupposed that detected voltages TV and SV of angle sensors 78and 79 are inputted (steps S401 and 402). It is judged that switch 32 isset on or off (step S403). If switch 32 is set off, output currents TIand SI of the solenoids are controlled through program R1 for HST110 andprogram R2 for HST 120 on the basis of both detected voltages TV and SV(steps S404 and S405). If brake pedal 51 is depressed so as to turn onswitch 32, HST controller 141 forcibly vanishes current TI of eitherenergized solenoid 61 a or 61 b in valve 61 and current SI of eitherenergized solenoid 62 a or 62 b in valve 62 (step S406 and S407), sothat both swash plates 111 a and 121 a are set to respective neutralpositions so as to stop the output rotations of respective hydraulicmotors 112 and 122.

In the embodiment of FIG. 19, brake device 58 acts at the same time whenboth HSTs 110 and 120 become almost neutral because switch 32 turns onwhen brake pedal 51 is depressed to position P2 which is close to themost depression.

Alternatively, brake pedal 51 may be set so that it turns on when pedal51 is depressed to position P1 shallower than position P2. Accordingly,in the process of depressing brake pedal 51 to the most depression,brake device 58 acts after both HSTs 110 and 120 become neutral. Namely,the neutralization of HSTs and the braking actuation are performed oneafter another according to operation of brake pedal 51.

Furthermore, by making the decreasing speed TI′ of current equal to orless than a fixed value according to the above-mentioned embodiment ofFIG. 16 so as to control the deceleration during neutralization of HSTs110 and 120, the vehicle is gradually decelerated and finally brakedperfectly, thereby reducing the shock in braking. Incidentally, onlycurrent TI for forward and backward traveling is referred to in theembodiment of FIG. 16. However, if the vehicle is braked in turning, thedecreasing speed of current SI for right and left turning may besimilarly decreased in proportion to the decreasing speed of current TIso that the vehicle can be decelerated while it keeps a constant turningradius.

Furthermore, when braking, main speed change lever 77 and steering wheel7 are not shifted to the respective neutral positions but maintained totheir places. Therefore, when brake pedal 51 is returned so as to turnoff switch 32, the outputs of HSTs 110 and 120 are controlled bycurrents TI and SI of the solenoids on the basis of both detectedvoltages TV and SV again, thereby re-creating the traveling speed justbefore depression of pedal 51. Even in this case, the vehicle speed canbe gradually increased to the original speed so as to reduce the shockin starting of the vehicle because the increasing speeds of currents arerestricted.

Alternatively, both HSTs 110 and 120 may be controlled in theirneutralization on the basis of the value detected by brake pedalposition sensor 31 instead of switch 32. It is presupposed, for example,that the neutralization of HSTs and the braking actuation are performedone after another, and a voltage BV detected by sensor 31 increasesaccording to depression of brake pedal 51 so that voltage BV becomes BV₀when pedal 51 is unpressed and it becomes BV₁ when pedal 51 is depressedto position P1. As shown in FIG. 20, in the state where detectedvoltages TV and SV of sensors 78 and 79 are inputted (step S501 andS502), voltage BV is measured (step S503). At the time of BV₀?BV<BV₁,currents TI and SI of the solenoids are controlled through program R1for HST 110 and program R2 for HST 120 on the basis of both detectedvoltages TV and SV (step S504 and 505). If BV reaches or exceeds BV₁ bydepressing brake pedal 51, HST controller 141 forcibly vanishes currentsTI and SI regardless of detections of sensors 78 and 79 (step S506 andS507), so that both swash plates 111 a and 121 a are set to therespective neutral positions so as to stop the output rotations ofrespective hydraulic motors 112 and 122.

In the case where the detection of sensor 31 is used, currents T1 and S1to the respective excited solenoids of valves 61 and 62 may be reducedcorrespondingly to the depressed position of brake pedal 51 until bothHSTs 110 and 120 become neutral. Therefore, the degrees of rotary speedreductions of left and right driving sprockets 11 can be adjusted bydepression of brake pedal 51. This structure may be used besides thestructure for neutralizing both HSTs 110 and 120 by changing switch 32.

Furthermore, if a selsyn motor for starting the engine is turned onelectricity only when both HSTs 110 and 120 are set in neutral bydepressing brake pedal 51 and brake device 58 is acting for braking, thesafety at starting of the engine can be assured.

FIG. 21 is a flow chart for controlling the selsyn motor in this way. Inthis case, switch 32 is disposed correspondingly to most-depressedposition P2 so as to decide whether brake device 58 is acting forbraking or not.

When the engine is stationary (a step S601), it is detected that brakepedal switch 32 is put on so as to confirm that brake device 58 isacting for braking (step S602), and it is confirmed that traveling HST110 is neutral on the basis of detected voltage TV of sensor 78 (stepS603). Then, the selsyn motor becomes possible to be turned onelectricity (step S604) so as to allow the engine to start.

While a preferred embodiment has been described, variations thereto willoccur to those skilled in the art within the scope of the presentinvention concepts which are delineated by the following claims.

INDUSTRIAL APPLICABILITY OF THE INVENTION

As mentioned above, by using the present invention, a vehicle such as acrawler traveling vehicle including a crawler tractor, which drivesrespective HSTs for traveling and steering, can have a compact, light,precise and economic electrically controlled system for controlling theHSTs without a complicated linkage interposed among the speed changeoperation tool such as a speed change lever, the steering operation toolsuch as a steering wheel, and the respective movable swash plates of theHSTs.

1. A hydraulically driven traveling vehicle, comprising: a pair of leftand right drive axles; a differential mechanism differentiallyconnecting said pair of drive axles to each other; a travelinghydrostatic stepless transmission; a steering hydrostatic steplesstransmission, wherein mutually opposite two-flow output rotations ofsaid steering hydrostatic stepless transmission are separatelytransmitted to said respective drive axles while output rotation of saidtraveling hydrostatic stepless transmission is transmitted to an inputsection of said differential mechanism, thereby making said vehicletravel and turn; a speed change operation tool manipulated by a driverso as to set each speed of said vehicle traveling forward and backward,wherein manipulated variable and direction of said speed changeoperation tool is converted into an electric signal; a steeringoperation tool manipulated by a driver so as to set each turning radiusof said vehicle turning left and right, wherein manipulated variable anddirection of said steering operation tool is converted into an electricsignal; a pair of electromagnetic solenoids serving as respective outputregulating means of said traveling hydrostatic stepless transmission andsaid steering hydrostatic stepless transmission, wherein an outputcurrent value of each of said electromagnetic solenoids is controlledbased on both the electric signals resulting from manipulation of saidspeed change operation tool and said steering operation tool so as tocreate an output rotary speed of each of said hydrostatic steplesstransmissions corresponding to said controlled output current value, abrake for braking both said left and right drive axles disposed in atransmission system to both said drive axles; and a brake operation toolfor acting said brake, wherein the output current values of said pair ofelectromagnetic solenoids are electrically controlled so as to vanishthe output current values of said traveling hydrostatic steplesstransmission and said steering hydrostatic stepless transmission,without changing positions of the speed change operation tool and thesteering operation tool, when said brake operation tool is operated at astroke so as to reach a prefixed neutral setting position, wherein bothof the output of the traveling hydrostatic stepless transmission and theoutput of the steering hydrostatic stepless transmission are controlledby both of the speed change operation tool and the steering operationtool, whereby the output of the traveling hydrostatic steplesstransmission and the output of the steering hydrostatic steplesstransmission are correlated with each other.
 2. The hydraulically driventraveling vehicle as set forth in claim 1, further comprising: a pair ofcrawler traveling devices disposed left and right sides of said vehiclerespectively, wherein said pair of crawler traveling devices haverespective driving sprocket shafts serving as said left and right driveaxles.
 3. The hydraulically driven traveling vehicle as set forth inclaim 1, further comprising: a steering wheel serving as said steeringoperation tool.
 4. The hydraulically driven traveling vehicle as setforth in claim 1, further comprising: a pair of hydraulicservomechanisms serving as respective output regulating means of saidtraveling hydrostatic stepless transmission and said steeringhydrostatic stepless transmission; and a pair of electromagneticproportional valves serving as respective hydraulic controlling meansfor said respective hydraulic servomechanisms, wherein said pair ofelectromagnetic solenoids are provided in said respectiveelectromagnetic proportional valves.
 5. The hydraulically driventraveling vehicle as set forth in claim 1, wherein the output currentvalue of said electromagnetic solenoid of said traveling hydrostaticstepless transmission is controlled on the basis of both the electricsignals resulting from manipulation of said traveling operation tool andsaid steering operation tool so that, while said steering operation toolis manipulated from a straight traveling setting position toward a limitposition for either left or right turning of said vehicle within acertain range, the output speed of said traveling hydrostatic steplesstransmission is almost fixed, and while said steering operation tool ismanipulated, beyond said certain range to said limit position, theoutput speed of said traveling hydrostatic stepless transmission isreduced so that the reduction degree thereof increases as said steeringoperation tool approaches said limit position.
 6. The hydraulicallydriven traveling vehicle as set forth in claim 5, wherein the outputcurrent value of said electromagnetic solenoid of said travelinghydrostatic stepless transmission is so controlled that a ratio ofoutput speed of said traveling hydrostatic stepless transmission whensaid steering operation tool being set in an arbitrary position to theoutput speed of said traveling hydrostatic stepless transmission whensaid steering operation tool being set in said straight travelingsetting position is fixed wherever said speed change operation tool isset.
 7. The hydraulically driven traveling vehicle as set forth in claim6, wherein the output current value of said electromagnetic solenoid ofsaid steering hydrostatic stepless transmission is controlled on thebasis of both the electric signals resulting from manipulation of saidspeed change operation tool and said steering operation tool so that aratio of output speed of said steering hydrostatic stepless transmissionwhen said steering operation tool being set in an arbitrary position tothe output speed of said traveling hydrostatic stepless transmissionwhen said steering operation tool being set in said arbitrary positionis fixed wherever said speed change operation tool is set.
 8. Thehydraulically driven traveling vehicle as set forth in claim 7, whereinthe output current value of said electromagnetic solenoid of saidsteering hydrostatic stepless transmission is controlled on the basis ofboth the electric signals resulting from manipulation of said speedchange operation tool and said steering operation tool so that, whilesaid speed change operation tool is set for creating a speed lower thana certain value, a ratio of output speed of said steering hydrostaticstepless transmission when said steering operation tool being set in anarbitrary position to the output speed of said traveling hydrostaticstepless transmission when said steering operation tool being in saidarbitrary position is larger than said fixed ratio.
 9. The hydraulicallydriven traveling vehicle as set forth in claim 7, wherein the outputcurrent value of said electromagnetic solenoid of said steeringhydrostatic stepless transmission is controlled on the basis of both theelectric signals resulting from manipulation of said speed changeoperation tool and said steering operation tool so that, while saidsteering operation tool is manipulated from said straight travelingsetting position to a limit position for either right or left turning ofsaid vehicle, a setting position of said steering operation tool forvanishing the rotary speed of said drive axle on inside of said vehiclein turning, which has been reduced from a speed during the straighttraveling of said vehicle, is fixed.
 10. The hydraulically driventraveling vehicle as set forth in claim 1, wherein an upper limit isprovided to a displacement speed of the output current value of saidelectromagnetic solenoid for said traveling hydrostatic steplesstransmission so that, when said displacement speed corresponding tomanipulation speed of said speed change operation tool becomes largerthan said upper limit, said output current value of said electromagneticsolenoid for said traveling hydrostatic stepless transmission varies atsaid upper limit of said displacement speed.
 11. The hydraulicallydriven traveling vehicle as set forth in claim 1, further comprising: afoot pedal serving as said brake operation tool.
 12. The hydraulicallydriven traveling vehicle as set forth in claim 1, further comprising: aswitch which changes when said brake operation tool is operated at thestroke so as to reach said neutral setting position, wherein the outputcurrent values of said electromagnetic solenoids are controlled bydetecting an electric signal as the change of said switch so as tovanish the output speeds of said traveling hydrostatic steplesstransmission and said steering hydrostatic stepless transmission. 13.The hydraulically driven traveling vehicle as set forth in claim 1,wherein the displacement speeds of output current values of saidelectromagnetic solenoids for resetting the output speeds of saidtraveling hydrostatic stepless transmission and said steeringhydrostatic stepless transmission when the stroke of said brakeoperation tool beyond said neutral setting position is reduced acrosssaid neutral setting position are held no more than respective certainvalues.
 14. The hydraulically driven traveling vehicle as set forth inclaim 1, wherein said neutral setting position of said brake operationtool is disposed at a position corresponding to a stroke of said brakeoperation tool which is smaller than a stroke thereof corresponding to abraking position for acting said brake.
 15. The hydraulically driventraveling vehicle as set forth in claim 14, wherein the displacementspeeds of output current values of said electromagnetic solenoids forvanishing the output speeds of said traveling hydrostatic steplesstransmission and said steering hydrostatic stepless transmission whensaid brake operation tool reaches said neutral setting position are heldunder respective certain values.
 16. The hydraulically driven travelingvehicle as set forth in claim 1, wherein an engine is allowed to startwhen said brake operation tool is operated at a stroke so as to reach abraking position and it is confirmed that said speed change operationtool is set at its neutral position.
 17. A hydraulically driventraveling vehicle comprising: left and right drive axles; a differentialmechanism differentially connecting the drive axles to each other; atraveling HST (hydrostatic stepless transmission) including a travelingHST pump with a swash plate and a traveling HST motor, wherein thetraveling HST pump is adapted to be coupled to an engine and controlsthe traveling HST motor by regulating output of the traveling HST pumpusing the swash plate; a steering HST including a steering HST pump witha swash plate and a steering HST motor, wherein the steering HST pump isadapted to be coupled to the engine and controls the steering HST motorby regulating output of the steering HST pump using the swash plate,wherein mutually opposite two-flow output rotations of said steering HSTare separately transmitted to said respective drive axles while outputrotation of said traveling HST is transmitted to an input section ofsaid differential mechanism, thereby making said vehicle travel andturn; a speed change operation tool manipulated by a driver so as to seteach speed of said vehicle traveling forward and backward, whereinmanipulated variable and direction of said speed change operation toolis converted into an electric signal; a steering operation toolmanipulated by a driver so as to set each turning radius of said vehicleturning left and right, wherein manipulated variable and direction ofsaid steering operation tool is converted into an electric signal; apair of electromagnetic solenoids configured to control the respectiveswash plates under predetermined rules for regulate respective outputsof said traveling HST and said steering HST, wherein an output currentvalue of each of said electromagnetic solenoids is controlled based onboth the electric signals resulting from manipulation of said speedchange operation tool and said steering operation tool so as to createan output rotary speed of each of said HSTs corresponding to saidcontrolled output current value; a brake for braking both said left andright drive axles disposed in a transmission system to both said driveaxles; and a brake operation tool for acting said brake, wherein theoutput current values of said pair of electromagnetic solenoids areelectrically controlled so as to vanish the output current values ofsaid traveling HST and said steering HST, without changing positions ofthe speed change operation tool and the steering operation tool, whensaid brake operation tool is operated at a stroke so as to reach aprefixed neutral setting position.
 18. The hydraulically driventraveling vehicle as set forth in claim 17, further comprising: a pairof hydraulic servomechanisms for manipulating said respective swashplates; and a pair of electromagnetic proportional valves forcontrolling said respective hydraulic servomechanisms, wherein said pairof electromagnetic solenoids are coupled to said respectiveelectromagnetic proportional valves.
 19. A hydraulically driventraveling vehicle with left and right drive axles, comprising: atraveling HST, output of which is regulated by an electromagneticsolenoid; a steering HST, output of which is regulated by anelectromagnetic solenoid; a speed change operation tool manipulated by adriver, which regulates both of the electromagnetic solenoids for thetraveling HST and for the steering HST; a steering operation toolmanipulated by the driver, which regulates both of the electromagneticsolenoids for the traveling HST and for the steering HST; a brake forbraking both said left and right drive axles disposed in a transmissionsystem to both said drive axles; and a brake operation tool for actingsaid brake, wherein output current values of said electromagneticsolenoids are electrically controlled so as to vanish output currentvalues of said traveling HST and said steering HST, without changingpositions of the speed change operation tool and the steering operationtool, when said brake operation tool is operated at a stroke so as toreach a prefixed neutral setting position, wherein both of the output ofthe traveling HST and the output of the steering HST are controlled byboth of the speed change operation tool and the steering operation tool,whereby the output of the traveling HST and the output of the steeringHST are correlated with each other.